A copolymer (hereinafter referred to also as an ETFE copolymer) having structural units derived from ethylene and structural units derived from tetrafluoroethylene, is excellent in heat resistance, chemical resistance, weather resistance, electrical insulation properties, mechanical properties, etc. and thus is used as a material for a coating layer of an electric wire in aircrafts, automobiles, industrial robots, etc.
However, an electric wire having a coating layer formed of an ETFE copolymer has such a problem that if the electric wire is held at a high temperature in a state of being bent, cracking is likely to occur in the coating layer. Recently, along with high performance of aircrafts, automobiles, etc., a coating layer of an electric wire is desired to have its nature against cracking (hereinafter referred to also as stress cracking resistance at a high temperature) improved so that even if the electric wire is held at a high temperature in a state of being bent, cracking is less likely to occur in the coating layer.
The following one has been proposed as an ETFE copolymer capable of improving stress cracking resistance of the coating layer at a high temperature.
An ETFE copolymer wherein the content of chlorine atoms is at most 70 ppm, the ratio (molar ratio) of structural units derived from tetrafluoroethylene/structural units derived from ethylene is from 40/60 to 70/30, structural units derived from other monomer(s) are contained in an amount of from 0.3 to 8 mol % based on all structural units, and the volume flow rate is from 0.01 to 1,000 mm3/sec. (Patent Document 1).